1. Field Of The Invention
The present invention relates to testing the bending strength of needles. In particular, the invention relates to an apparatus and method to determine the bending strength and ductility of a curved surgical needle by precision application of known bending moments and by monitoring the resultant forces and moments generated.
2. Description of Related Art
Various methods and apparatus for measuring the bending strength of surgical needles are known. Generally, a needle to be tested is fixed at one end in a device for measuring stress and strain. A bending force is then applied to the needle. The amount of force necessary to achieve bending of the needle is then measured. In particular, with curved surgical needles, the accurate and consistent application of a bending moment has proven to be difficult, particularly due to the curvature of the needle. For example, in order to apply a bending moment which is consistent over a period of time and rotational span, the force and the direction of the moment arm must be maintained with consistency and extreme precision. Furthermore, this precision must be maintained throughout the bending action of the needle.
There are known methods to measure stress, strain and other material strength characteristics of a sample of material These methods include manual manipulation of the material to subjectively feel the strength. This method is not uniform and does not indicate force requirements for deformation of the material. Extensometers measure minute deformations of a test specimen caused by tension, compression, bending or twisting.
It is significant to obtain both strain and deflection readings simultaneously because both types of strain, e.g., axial strain and bending deflection will occur even when stress is applied in only one direction. Therefore, in order to fully evaluate a specimen, both stress measurements are needed. Some techniques for determining axial and bending moment values may interfere with each other and prevent simultaneous accurate measurement.
In dynamic testing apparatus, a specimen of material is subject to a variety of test loads. These may include tension, compression and torsion. A major problem with many apparatus is that the devices which secure or clamp the test specimen have created unwanted, nonuniform stresses and strains on the specimen because of the manner in which the specimen is secured. For example, gripping of the ends of a specimen will introduce compression tension or shear forces, resulting in errors in the test results. If a sample is not perfectly aligned with the two gripping devices, non uniform loading of the sample will result.
Difficulties encountered in prior attempts to test the bending strength of curved needles are shown in FIG. 1 which illustrates a known device 1 for measuring the bending strength of a curved surgical needle 2. The barrel end 3 of the needle is placed in the needle gripping fixture 4 of the instrument. Rotary actuator 5 has a peg 6 for contacting the needle 2. To measure the bend strength of needle 2, actuator 5 is rotated in the direction of arrow 7 about central axis 8. Peg 6 contacts the concave side of the needle and is supplied with sufficient force to bend the needle. The applied force is measured by appropriate force sensing instrumentation.
Bending stresses are developed in the needle around the area contacted by the gripping fixture. However, it can be seen that the point of application of the force by peg 6 will change relative to the needle, causing inaccuracies in the resultant measured stresses. These stresses are non-uniform in magnitude and direction, and such non-uniformity introduces a source of error in the measurement of the bending strength of the needle.
I have invented an apparatus and method for securing and testing needles for determining the bending strength and ductility in a manner which provides improved accuracy and precision, with resultant uniform and accurate measurements.